This invention relates to an optical correlation-type velocity measurement apparatus and, particularly an apparatus which makes it possible to prevent a change in the size and shape of light-receiving spots, a change in the measurement spot spacing of front and rear sensors, etc. The present invention also makes it possible and to shorten the distance between the sensors and further enables the frictional coefficient of a road surface to be measured at the same time as velocity measurement.
In general, a correlation-type speedometer is used to detect velocity, such as flow velocity and vehicle velocity.
The principle of a correlation-type speedometer used conventionally will be described with reference to FIG. 1.
For example, as shown in FIG. 1, optical sensors 1 and 3 are provided on a vehicle body 5 with a spacing l therebetween in the direction of travel indicated by the arrow, and the respective output waveforms of the sensors obtained by sensing light reflected from a measured surface 7 (i.e., the surface of a road) are sampled at a period .sub..DELTA. t. If the series of sampled values so obtained is represented by x.sub.p, y.sub.p (p=0, 1, 2, . . . N-1), respectively, a cross correlation function r.sub.xy (k) of these two waveforms will be expressed as follows: ##EQU1## If r.sub.xy (k.sub.m) represents the maximum value of r.sub.xy (k), then the stagger time between the two waveforms will be .sub..DELTA. t.multidot.k.sub.m. Accordingly, the travelling velocity V will be expressed as follows: EQU V=l/.sub.66 t.multidot.k.sub.m
In almost all cases the shape of the field of view of the sensors used in a correlation-type speedometer is circular, and shapes other than this are not particularly stipulated. However, it is known that the smaller the width of the field of view in the forward direction, the higher the frequency component of the signal obtained at the same velocity. In order to achieve an accurate measurement of velocity, it is necessary to sample a minimum of several periods of the signal. The higher the signal frequency, the more sampling time can be shortened. In order to shorten sampling time by means of a such a characteristic, the sensor field of view should be reduced. If this is done when the field of view is circular, however, there is a higher probability that the fields of view of the two sensors will traverse different areas, thereby inviting a decline in correlativity and diminishing measurement precision. Furthermore, by reducing the area of the field of view, there is greater susceptibility to the effects of external disturbances as caused by the motion of snow flakes or water droplets which come between the sensors and the object being measured. This is a fatal flaw in an ABS (anti-skid braking system) which is designed to be effective for snow-covered and wet roads.
FIG. 2 illustrates a speedometer using a conventional spatial filter-type detection system. The arrangement includes sensors 11, 13, a focal plane 15, a slit or pin-hole 17 and a lens principal plane 19. Numeral 21 denotes a road surface, and numerals 23, 25 designate spot optical paths.
If the device shown in FIG. 2 is arranged so that the lens focal points are placed at the position of the slit or pin-hole 17, as shown in FIG. 2, only the parallel light incident upon the lenses will will pass through the slit or pin-hole to be detected. Sensor 11 senses only the parallel light along optical path 25, and sensor 13 senses only the parallel light along optical path 23. Accordingly, spot width w will not change even if the distance between the road surface and the lenses varies. When the outputs of sensors 11, 13 are combined, a change in reflected light caused by sensor pitch width and irregularities in the same road surface is emphasized, while a change in reflected light due to other irregularities is random with respect to the sensors 11, 13 and therefore is cancelled out. As a result, velocity can be detected from the extracted output frequency.
In a case where sensors are mounted on an automotive vehicle and a correlation function is computed to determine velocity relative to a road surface, there is a change in the shape and size of the light-receiving spots of the front and rear sensors when the measurement distance varies due to bouncing of the vehicle body or unevenness in the road surface. Since this change in spot shape and size means that the front and rear sensors will not scan the same points on the road, agreement between the sensor output waveforms diminishes and correlativity declines. When the measurement distance fluctuates, moreover, the distance between the spots essentially varies, leading to the problem of measurement error. In this regard, the size and shape of the sensor spot will not change if sensors arranged as shown in FIG. 2 are applied to a correlating-type speedometer. However, since only parallel light that passes through the focal point is received, a problem is that the amount of light is small, with a low S/N ratio resulting. In order to raise the S/N ratio, a high-output light source is required, which in turn leads to problems relating to power consumption, size and service life.
Furthermore, if the surface under measurement is a scattering reflective surface, measurement is possible with the conventional correlation-type speedometer but this will not be the case if the reflecting surface is mirror-like, as can happen if the surface is covered with water droplets or a water film. This point will now be described in connection with FIGS. 3 and 4.
In a case where reflected light originating from the same light source 41 is received by sensors 42, 43 provided on a vehicle body 40 one in front of the other in the direction of travel, as shown in FIG. 3, assume that incident light 45 impinges at an angle .alpha. at a point A on a water surface 44, and that light 46 reflected from the water surface at the angle .alpha. is sensed by the sensor 42.
Next, assume that the vehicle travels the distance l between the sensors and that .beta. is now the angle of incidence of light 47 received at point A from the light source 41. In such a case reflected light 48 from point A cannot be sensed by the sensor 43.
When a scattering reflective surface is involved, light is reflected in all directions even if the angle of incidence differs somewhat, and this makes it possible for the sensors 42, 43 to receive light reflected from the same point despite a difference in the angle of incidence. With a mirror-like surface, however, light is reflected only in a direction where the reflective angle is equal to the angle of incidence, so that light received from the same light source by one sensor will not be received by the other. As a result, the correlativity of the two sensor output waveforms diminishes and measurement may become impossible in a worst-case scenario.
In an attempt to solve this problem, a set-up of the kind shown in FIG. 4 may be adopted, in which light sources 51, 52 are provided for respective sensors 53, 54 and these elements are arranged in such a manner that the relative positional relationship between light source 51 and sensor 53 is exactly the same as that between light source 52 and sensor 54. With this arrangement, light reflected from the same point can be sensed even if the surface under measurement is a mirror surface.
However it is necessary that the set-up depicted in FIG. 4 be arranged so that each sensor does not pick up reflected light originating from the other sensor's light source. This means that the distance between the two sensors cannot be made very short and, thus sampling time is prolonged. If distance between the sensors is enlarged, the fields of view of the front and rear sensors pass along different locations, as when the vehicle rolls relative to the direction of travel, as a result of which waveform correlativity declines.
In addition, 2-wheel/4-wheel drive changeover, center differential locking control and braking control are performed upon comparing engine driving force and the force acting upon the vehicle wheels. In calculating the force acting upon the wheels in such case, the condition of the road surface is ascertained, and the coefficient of friction is determined, based on the light reflected from the road surface. However, where vehicle drive control and braking control are carried out by measuring actual velocity and the coefficient of friction of the road surface simultaneously, it is required that identical optical sensors be separately provided for sensing velocity and for measuring the coefficient of friction, and that a system be provided for processing each of the sensor output signals. This arrangement would involve a great deal of waste and pose problems in terms of sensor installation space and cost.